Go proactive to protect against glyphosate-resistant weeds

Jason L. Jenkins, University of Missouri | Jan 04, 2007

When it comes to the development of glyphosate-resistant weeds, producers have two choices: Pay more now or pay much more later.

“It might be more expensive upfront, but working to prevent glyphosate-resistant weeds from ever developing is more economical in the long run,” said Ray Massey, an agricultural economist at the University of Missouri. Massey spoke to the MU Crop Management Conference in Columbia, Mo., about data gathered from a 2006 MU field trial of glyphosate-resistant tall waterhemp in western Missouri.

Massey determined that proactive management is a better economic choice than reactive management if resistance was to develop in a field within 26 years.

“That means that even if resistance didn’t occur until year 25, it would cost less to spend a little extra money each year for 25 years than to wait until resistance developed and then pay to control it,” he told the audience of about 60 certified crop advisers and farmers.

Massey used data collected by MU weed scientist Kevin Bradley, who earlier this year confirmed tall waterhemp as Missouri’s third glyphosate-resistant weed and sixth such weed in the U.S.

The western Missouri field where the resistant waterhemp developed had been in continuous soybeans since 1996. Glyphosate had been used continuously.

“It’s a pretty nasty beast we’re dealing with on this site,” Bradley said. “We were able to get control with some pretty expensive programs, but it’s pretty bad.

“I freely admit I’m trying to scare producers with this scenario into doing the right thing.”

He added that weed scientists always expected it would be better to try to prevent glyphosate resistance than to let it happen. “Now we’ve learned that, from an economic standpoint, what we were saying was right,” Bradley said. “We have the numbers to confirm it.”

Missouri is the first and only state where three glyphosate-resistant weeds — tall waterhemp, common ragweed and horseweed — have been identified. Bradley said Missouri holds this dubious honor because of the large number of acres planted to continuous soybeans, 90 percent of which use a glyphosate system for weed control.

“Resistant weeds are not something we create,” Bradley said. “They exist naturally and we select for them by using the same herbicide continuously.”

It’s not uncommon to find producers who believe “that resistant weeds aren’t a problem until they occur on my land,” he said. “There’s a belief that a silver bullet will come along, but there isn’t another silver bullet. I just don’t see it happening.”

According to Bradley, preventing glyphosate resistance is best achieved by rotating to herbicides with different modes of action in a field. This can be done by using pre-emergence herbicides followed by glyphosate, or by rotating glyphosate with other post-emergence herbicides in alternate years.

“While we know it’s important to be proactive, what we can’t say yet is how often we need to be proactive with a glyphosate system,” he said. “It may need to be an every-other-year application of a herbicide with a different mode of action, or it may only need to be every third year.”

The use of glyphosate increased dramatically in the mid-1990s with the introduction of crops genetically modified to tolerate the herbicide. In addition to soybeans, varieties of corn, cotton, canola, sorghum and alfalfa also have been developed to allow the post-emergence use of the herbicide.

To date, scientists have confirmed nine glyphosate-resistant weeds worldwide. In addition to the three in Missouri, they include buckhorn plantain, goosegrass, hairy fleabane, Italian ryegrass, palmer amaranth and rigid ryegrass.